1. Field of the Invention
The present invention relates to a drive circuit for driving a power semiconductor device and for detecting occurrence of an abnormality in the power semiconductor device.
2. Description of Related Art
FIG. 24 is a block diagram showing the structure of a prior art drive circuit for driving a power semiconductor device. In the figure, reference numeral 1 denotes an IGBT (Insulated Gate Bipolar Transistor) that is a power semiconductor device, reference numeral 2 denotes a control logic for sending a gate instruction to cause the IGBT 1 to make a transition to a closed state to a buffer 3 when receiving a turn-on instruction from outside the drive circuit, and for sending a gate instruction to cause the IGBT 1 make a transition to an open state to the buffer 3 when receiving a turn-off instruction from outside the drive circuit, reference numeral 3 denotes the buffer for driving the IGBT 1 according to the gate instruction sent thereto from the control logic 2, reference numeral 4a denotes an on-state gate resistor, reference numeral 4b denotes an off-state gate resistor, reference numeral 5 denotes a diode having a cathode connected with a collector of the IGBT 1, and an anode connected with a collector voltage detector 6, and reference numeral 6 denotes the collector voltage detector for delivering an abnormality signal to the control logic 2 when the anode of the diode 5 has a voltage larger than a reference voltage.
Next, a description will be made as to an operation of the prior art drive circuit for driving a power semiconductor device. When receiving a turn-on instruction from outside the drive circuit, the control logic 2 sends a gate instruction to cause the IGBT 1 to make a transition to a closed state (referred to as “become turned-on” from here on) to the buffer 3. When receiving the gate instruction to cause the IGBT 1 to become turned-on from the control logic 2, the buffer 3 amplifies the gate instruction and sends the amplified gate instruction to the gate of the IGBT 1 by way of the on-state gate resistor 4a. As a result, the IGBT 1 becomes turned-on.
When receiving a turn-off instruction from outside the drive circuit, the control logic 2 sends a gate instruction to cause the IGBT 1 to make a transition to an open state (referred to as “become turned-off” from here on) to the buffer 3. When receiving the gate instruction to cause the IGBT 1 to become turned-off from the control logic 2, the buffer 3 amplifies the gate instruction and sends the amplified gate instruction to the gate of the IGBT 1 by way of the off-state gate resistor 4b. As a result, the IGBT 1 becomes turned-off.
When the IGBT 1 becomes turned-on, the voltage of the collector of the IGBT 1 decreases and the diode 5 becomes turned-on. As a result, the anode of the diode 5 has a voltage equal to the sum of the on-state voltage of the IGBT 1 and the on-state voltage of the diode 5 with respect to the voltage of the emitter of the IGBT 1. When a collector current flowing in the IGBT 1 then increases, the collector voltage of the IGBT1 increases too and the anode voltage of the diode 5 increases with the increasing collector voltage of the IGBT 1. Therefore, by monitoring the anode voltage of the diode 5 within a time period during which the IGBT 1 is turned on, it is possible to detect a short circuit that can occur when a large current flows in the IGBT 1, for example, because the anode voltage of the diode 5 rises greatly.
Then, the collector voltage detector 6 monitors the anode voltage of the diode 5, and, when the anode voltage becomes larger than a reference voltage, delivers an abnormality signal to the control logic 2. When receiving the abnormality signal from the collector voltage detector 6, in order to prevent the IGBT 1 from being destroyed due to a large current flowing in the IGBT, the control logic 2 sends a gate instruction to turn off the IGBT 1 to the buffer 3 so that the buffer 3 cuts off the large current.
A problem encountered with a prior art drive circuit constructed as above for driving a power semiconductor device is that when the drive circuit is applied to a high-voltage IGBT, a number of high-voltage diodes 5 need to be connected in series and this results in an increase in the total cost of the drive circuit and a reduction in the reliability of the drive circuit. Furthermore, in a high-voltage IGBT, the collector-emitter voltage doesn't change to a steady-state value (i.e., a value acquired according to the static characteristics of the IGBT) instantaneously, but reaches the steady-state value at the expiration of a certain transition time interval after the IGBT has been turned on (according to circumstances, the collector voltage might still be tens of volts at the expiration of 10 or more microseconds after the IGBT has been turned on). Another problem is that to distinguish normal turning-on from a short circuit, the above-mentioned transition time period needs to be masked, so short-circuit detection by the collector voltage detector 6 is remarkably delayed and therefore the IGBT 1 cannot be protected.